Ceramic block filters have become very popular for a variety of reasons including their small size, superior performance over lumped elements, and their manufacturability. Most ceramic block filters are comprised of parallelepiped blocks of dielectric material through which at least one hole extends. The surfaces of the block, including the inner surface of the hole or holes, with the exception of the top surface, are coated with a metallic or other conductive material. The conductive material lining the hole, in combination with conductive material coating the block, is electrically equivalent to a shorted length of transmission line of specific electrical length at some particular frequency.
Many of these block filters, the top surfaces of which are substantially uncoated, use printed capacitors on this top surface to control the frequency characteristics of this filter. These capacitors are typically formed by appropriately patterning conductive material on the top surface such that conductive areas that are spaced from one another on the top surface form capacitances the values of which, albeit small, can be used to adjust the bandpass or cutoff frequency of the filter.
It is well known that the top or open circuit end of the coaxial resonators in a block resonator have strong electric fields radiating therefrom which electric field may adversely affect circuitry surrounding the filter in a radio or other communication device. In conventional filters, (i.e., lumped-element filters using discreet inductors, resistors and capacitors) electric field radiation is minimized by enclosing the filter components in a metal housing which housing is typically coupled to the ground or reference potential for the circuit. In prior art block filters, electrical field radiation from a block filter is reduced by enclosing or otherwise confining the open or top surface of the filter in a metal bracket which is typically attached to the smooth exterior sides of the block by soldering. So-called surface mount block filters use an "L" or "U" shaped metal shield to contain the radiation.
While a bracket can provide effective shielding of electric fields emitted from the open or top surface of the block filter, it is difficult to attach to an otherwise smooth surface of the block and it also adds to the height and overall size of the filter because the shield uses the blocks outer edges for alignment while it is being soldered to the block.
In many radio communication devices, such as small low profile portable cellular telephones lightweight and compact size are becoming increasingly more popular in the market. Frequently, the thickness of such a unit is determined at least in part by the height and size of the ceramic filters on the circuit boards that comprise the phone. A low profile, surface mount ceramic block filter that minimizes the height associated with a shielded filter, and eases the assembly and manufacturing of such a filter would be an improvement over the prior art.